Frequently, excessive voltage is applied across service lines which deliver power to residences and commercial and institutional facilities. Such excess voltage or voltage spikes may result from lightning strikes, for example. The voltage surges are of particular concern in telecommunications distribution centers, hospitals and other facilities where equipment damage caused by voltage surges and resulting down time may be very costly.
Typically, one or more varistors (i.e., voltage dependent resistors) are used to protect a facility from voltage surges. Generally, the varistor is connected directly across an AC input and in parallel with the protected circuit. The varistor has a characteristic clamping voltage such that, responsive to a voltage increase beyond a prescribed voltage, the varistor forms a low resistance shunt path for the overvoltage current that reduces the potential for damage to the sensitive components. Typically, a line fuse may be provided in the protective circuit and this line fuse is blown or weakened by the essentially short circuit created by the shunt path.
Varistors have been constructed according to several designs for different applications. For heavy duty applications (e.g., surge current capability in the range of from about 60 to 100 kA) such as protection of telecommunications facilities, block varistors are commonly employed. A block varistor typically includes a disk shaped varistor element potted in a plastic housing. The varistor disk is formed by pressure casting a metal oxide material, such as zinc oxide, or other suitable material such as silicon carbide. Copper, or other electrically conductive material, is flame sprayed onto the opposed surfaces of the disk. Ring shaped electrodes are bonded to the coated opposed surfaces and the disk and electrode assembly is enclosed within the plastic housing. Examples of such block varistors include Product No. SIOV-B860K250 available from Siemens Matsushita Components GmbH & Co. KG and Product No. V271BA60 available from Harris Corporation.
Another varistor design includes a high energy varistor disk housed in a disk diode case. The diode case has opposed electrode plates and the varistor disk is positioned therebetween. One or both of the electrodes include a spring member disposed between the electrode plate and the varistor disk to hold the varistor disk in place. The spring member or members provide only a relatively small area of contact with the varistor disk.
The varistor constructions described above often perform inadequately in service. Often, the varistors overheat and catch fire. Overheating may cause the electrodes to separate from the varistor disk, causing arcing and further fire hazard. There may be a tendency for pinholing of the varistor disk to occur, in turn causing the varistor to perform outside of its specified range. During high current impulses, varistor disks of the prior art may crack due to piezoelectric effect, thereby degrading performance. Failure of such varistors has led to new governmental regulations for minimum performance specifications. Manufacturers of varistors have found these new regulations difficult to meet.